This is Q. 12 from the Manhattan LSAT Prep Test 32. I did not originally answer this question because I think that LSAT questions generally shouldn't be used in GMAT prep. I don't know of any GMAT question that asks someone to identify the conclusion of an argument.

However, since you asked, I will delve into finding the conclusion of an argument. Generally speaking, the conclusion will:

1. Be accompanied by indicator words such as therefore, thus, hence, clearly, etc.
2. Include modal verbs such as must, should, would, will, etc.
3. Express an opinion or a claim rather than a fact.
4. Contain some form of the verb to be.
5. Be supported by other portions of the text (the why? test).

Based on these criteria, we can see that the top candidate for the conclusion is:

...using the power of artistic representation solely to preserve and reinforce objects that would exist even without artists is an ironic waste.

It contains a modal verb.
It expresses an opinion.
It contains the word is, a form of the verb to be.

However, it fails the why? test. There are no other portions of the argument that support this choice.

In reality, the conclusion of this argument is contained in the first sentence, but it must be completely rephrased to make it into a recognizable conclusion. The key word is mistakenly. When the argument says "Many artists mistakenly think that models need be taken only from outside the psyche" we can rephrase this as: It is a mistake to think that models need be taken only from outside the psyche. This is nicely summed up by answer choice (A).

However, I still think that this problem is far too subtle and difficult to be a good representation of a typical GMAT problem. Two-thirds of GMAT problems are strengthen, weaken, and assumption questions. Until a student has mastered those three types, it is generally a waste of time to delve into the more difficult question types.